JPH1053882A - Etching method for silicon - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for easily and rapidly etching silicon by using high-purity nitric acid. SOLUTION: Nitrogen monoxide or nitrogen dioxide is formed in a nitric acid bath contg. a reducing agent and the nitrogen monoxide is oxidized to generate gaseous nitrogen dioxide from the nitric acid bath. The gaseous nitrogen dioxide and the gaseous hydrogen fluoride, more adequately, the gaseous hydrogen fluoride produced by evaporation of the gaseous hydrogen fluoride dissolved in the nitric acid bath are mixed. The silicon is thereafter subjected to etching in the gaseous mixture composed of the resulted nitrogen dioxide and the hydrogen fluoride or the aq. soln. thereof.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for etching silicon, and more particularly to a method for easily and quickly chemically etching silicon with low contamination.

[0002]

2. Description of the Related Art Silicon is used as a material for a semiconductor substrate, and with the improvement in the degree of integration of semiconductors, it is necessary to reduce the contained impurities to a very small amount (pg / g to ng / g). Representative examples of such impurities include metal elements and organic substances. Since these segregate on the surface of silicon, they can be removed by chemically etching the surface of silicon and are widely used.

[0003] Hydrogen fluoride and an oxidizing agent are generally used as reagents for chemically etching silicon.
Nitric acid is widely used as an oxidizing agent. The reagent used for etching needs to be as high in purity as silicon, because impurities contained in it contaminate silicon.

Since a high-purity nitric acid reagent is expensive, as a method of performing chemical etching using normal-purity nitric acid, a nitric acid bath is heated to about 60 ° C. to generate nitrogen dioxide gas. And hydrogen fluoride gas, and then etching silicon in the obtained mixed gas of nitrogen dioxide and hydrogen fluoride or an aqueous solution thereof. In this method, since nitric acid is separated from impurities contained when it becomes nitrogen dioxide gas, etching can be performed without contaminating silicon with impurities of the reagent. In this case, since it is required to use hydrogen fluoride having a high purity, hydrogen fluoride is also dissolved in the above-mentioned nitric acid bath and gasified in the same manner, so that nitrogen dioxide is efficiently dissolved.
It is common practice to obtain a hydrogen fluoride mixed gas.

[0005]

In the above-mentioned etching method involving the vaporization of nitric acid, the generation of nitrogen dioxide gas is carried out by heating, so that it takes a long time for decomposition. Therefore, development of a method for performing such etching simply and quickly has been desired.

[0006]

Means for Solving the Problems The present inventors have intensively studied in view of the above problems. As a result, it has been found that the above problem can be solved if a reducing agent is contained in a nitric acid bath to generate nitrogen dioxide, and the present invention has been completed.

That is, according to the present invention, nitric oxide or nitrogen dioxide is generated in a nitric acid bath containing a reducing agent, and the nitric oxide is oxidized to generate nitrogen dioxide gas from the nitric acid bath. A method for etching silicon, comprising mixing a gas and a hydrogen fluoride gas, and then etching the silicon in the obtained mixed gas of nitrogen dioxide and hydrogen fluoride or an aqueous solution thereof.

In the present invention, the concentration of nitric acid to be used can be used without any limitation. However, if it is thin, the rate of generation of nitrogen dioxide gas becomes slow. It is desirable that In addition to the water, sulfuric acid may preferably be contained in an amount of 50% by weight or less.

On the other hand, as the reducing agent contained in the above-mentioned nitric acid bath, a reducing agent which reduces nitric acid to produce nitrogen dioxide or nitric oxide is used without any limitation. Specifically, it is a reducing agent having an oxidation-reduction potential of 0.94 eV or less, metals such as metallic copper, metallic zinc and metallic tin, reducing organic compounds such as ascorbic acid and oxalic acid, and salts such as iron (II) chloride. When the nitric acid bath and the hydrofluoric acid bath are the same bath, examples thereof include silicon and metal tungsten. Among them, it is preferable to use metallic copper or silicon because high-purity ones are easily available and a reduction reaction easily occurs. Here, the addition amount of the reducing agent is not particularly limited, but considering the generation efficiency of nitrogen dioxide, nitric acid 1
0.2 parts by weight or more, preferably 1-1
It is preferably 0 parts by weight.

In the nitric acid bath containing the reducing agent, nitric acid is reduced to produce nitrogen dioxide or nitric oxide. For example, when the reducing agent is metallic copper, if the nitric acid bath is a concentrated nitric acid bath, nitrogen dioxide is directly generated, and if the nitric acid bath is a dilute nitric acid bath, nitric oxide is generated. Thus, even when the above-mentioned nitric oxide is generated, it is immediately oxidized to nitrogen dioxide by dissolved oxygen in a nitric acid bath or oxygen in a gas phase in contact with the nitric acid bath. Nitrogen dioxide gas is generated from the nitric acid bath. Here, the gas phase in the case of oxidizing the nitrogen monoxide is not limited as long as it contains oxygen and does not cause another reaction with nitrogen dioxide or nitric oxide, but usually, air is used. Is common. Of course,
It is preferable to use a clean gas phase. Further, the temperature of the nitric acid bath is not particularly limited, and a temperature at which the reduction reaction of nitric acid easily proceeds depending on the type of the reducing agent may be appropriately set, but is usually selected from 25 to 90 ° C.

In the present invention, high-purity nitrogen dioxide gas can be easily and quickly obtained by generating nitrogen dioxide gas as needed from the nitric acid bath by a reduction reaction. In the present invention, this nitrogen dioxide gas is then mixed with hydrogen fluoride gas. Although any hydrogen fluoride gas may be used, it is preferable to use high-purity hydrogen fluoride gas. In order to obtain such high-purity hydrogen fluoride gas simply and inexpensively, the hydrogen fluoride gas is dissolved. It is desirable to use what has been vaporized from the bath. In such a bath for dissolving hydrogen fluoride, the amount of hydrogen fluoride to be dissolved is not particularly limited, but is 10% by weight or more, preferably 20 to 50% by weight in consideration of a sufficient amount of hydrogen fluoride generated. It is preferred that Also, the bath for dissolving hydrogen fluoride is usually
Suitably, it is an aqueous solution of hydrogen fluoride, ie a hydrofluoric acid bath.

Further, in the present invention, in order to efficiently obtain the above-mentioned mixed gas of nitrogen dioxide and hydrogen fluoride, it is particularly preferable that the bath for dissolving the hydrogen fluoride is the same as the nitric acid bath. That is, it is most efficient to dissolve hydrogen fluoride in a nitric acid bath containing the reducing agent, to simultaneously generate nitrogen dioxide gas and hydrogen fluoride gas, and to obtain a mixed gas of both. The present invention also includes an embodiment in which both gases are mixed simultaneously with generation.

In the present invention, the mixing ratio of the two in the mixed gas of nitrogen dioxide and hydrogen fluoride is not particularly limited, but considering the decomposability of silicon, nitrogen dioxide is preferably 2 to 95%. Is preferably 10 to 80%, and hydrogen fluoride is preferably 5 to 98%, preferably 20 to 90%. Therefore, when hydrogen fluoride is obtained by vaporizing from the dissolving bath, it is desirable to adjust the amount of vaporization to be a suitable desired amount of the hydrogen fluoride.

Next, the mixed gas of nitrogen dioxide and hydrogen fluoride obtained as described above is used for etching silicon. This etching can be sufficiently performed by exposing silicon to the nitrogen dioxide / hydrogen fluoride mixed gas, but in order to perform etching more uniformly, the nitrogen dioxide / hydrogen fluoride mixed gas is dissolved in water to form an aqueous solution. It is preferable to immerse the silicon in this. At that time, nitrogen dioxide
The dissolved amount of the hydrogen fluoride mixed gas is preferably 0.2% by weight or more, preferably 5 to 20% by weight as a total amount of both in order to obtain stronger silicon decomposability. The temperature of the mixed gas of nitrogen dioxide and hydrogen fluoride and the temperature of the aqueous solution during etching are not particularly limited, but are usually selected from 0 to 80 ° C. The etching time may be arbitrarily set until the desired amount of the silicon surface to be processed is removed, but generally, it is 5 to 7200.
Adopted from minutes.

In the present invention, silicon to be etched may be any of polycrystal, single crystal, and amorphous. Of these, polycrystals and single crystals are preferred.

Further, the etching method of the present invention can be applied without limitation when it is necessary to remove these silicon surfaces. Specifically, there is a case where the contamination of the surface layer is removed, and a case where the purity of silicon is analyzed by analyzing a dissolved substance dissolved by etching. Among them, it is particularly preferable to employ the above when measuring the analysis of silicon. Specifically, this analysis is preferably performed by inductively coupled plasma mass spectrometry, inductively coupled plasma emission analysis, or atomic absorption analysis.

The above-described method of the present invention will be further described with reference to FIG. In FIG. 1, reference numeral 1 denotes a container main body for performing etching, and a nitric acid bath 2 containing a reducing agent is provided at the bottom. In addition, this nitric acid bath 2
Has hydrogen fluoride dissolved therein. This dissolution of hydrogen fluoride can be carried out by providing another dissolving bath. Further, the etching vessel 1 is placed on a heating element 3 and is heated as necessary so that nitrogen dioxide gas and hydrogen fluoride gas are generated at desired production rates.

In the etching vessel 1, nitric acid is reduced by the reducing agent contained in the nitric acid bath 2 to generate nitrogen dioxide gas or nitrogen monoxide gas. Even when nitric oxide gas is generated, the gas phase of the etching vessel 1 is filled with an oxygen-containing atmosphere such as air, so that the nitric oxide gas is immediately oxidized to generate nitrogen dioxide gas. . Hydrogen fluoride is also vaporized from the nitric acid bath 2. Thus, the gas phase of the etching container 1 is filled with the mixed gas of nitrogen dioxide and hydrogen fluoride. In the present invention, the silicon to be treated is exposed to the gas phase, or an inner container 4 filled with water is further provided in the etching vessel 1, and the mixed gas of nitrogen dioxide and hydrogen fluoride is dissolved in the aqueous phase. By soaking silicon 5 in this,
Silicon can be etched well.

[0019]

According to the present invention, the generation of nitrogen dioxide is promoted by the action of the reducing agent added to the nitric acid bath. As a result, a sufficient amount of high-purity nitrogen dioxide gas or nitric acid is supplied to silicon. Silicon can be etched at a favorable rate by using the mixed gas and the hydrogen fluoride gas.

[0020]

The present invention will be described below with reference to examples and comparative examples, but the present invention is not limited to these examples.

Example 1 Polycrystalline silicon was etched by an etching apparatus shown in FIG. 1 installed in a constant temperature chamber at 25 ° C. Nitric acid bath 2
Was prepared by adding 1 g of polycrystalline silicon as a reducing agent to 150 ml of an aqueous solution adjusted to 35% by weight of nitric acid and 25% by weight of hydrogen fluoride. The heating element 3 is a hot plate having a surface temperature of 130 ° C., and the liquid temperature in the nitric acid bath is 70 ° C. Polycrystalline silicon to be etched
Four 20-g masses were placed in the inner container 4 containing 60 ml of water. The etched amount was determined by taking out polycrystalline silicon and measuring the weight with a balance.

The time-dependent changes in the etching amount obtained by the above etching are shown in Table 1 together with the standard deviation as an average value when the same operation was performed three times. The results are shown in FIG. Etching amount is 2
It increased rapidly after time and about 1 g could be etched after 3 hours.

A blank test was performed by the same operation except that the polycrystalline silicon to be etched was not put in the inner container 4. Five hours after the etching, the increase in iron in the water in the inner container was 0.001 ng / ml or less, indicating that the dissolved nitrogen dioxide / hydrogen fluoride mixed gas was extremely high in purity.

[0024]

[Table 1]

Comparative Example 1 The procedure of Example 1 was repeated, except that no polycrystalline silicon was added to the nitric acid bath. In this etching, the etching amount was about 0.01 g in 3 hours as shown in Table 2 and FIG. 3, and it took 6 hours to etch about 1 g.

A blank test was performed by the same operation except that the polycrystalline silicon to be etched was not put in the inner container 4. The amount of increase in iron in the water in the inner container after 5 hours of etching was 0.001 ng / ml or less.

[0027]

[Table 2]

Example 2 Example 2 was carried out in the same manner as in Example 1 except that 1 g of metallic copper was used as a reducing agent added to the nitric acid bath. In this etching, the etching amount was about 1 g in 3 hours as shown in Table 3, and 3 hours was enough to etch about 1 g.

[0029]

[Table 3]

Example 3 Example 3 was carried out in the same manner as in Example 1 except that water was not added to the inner container. In this etching, the etching amount was about 1.8 in 3 hours as shown in Table 4.
g.

[0031]

[Table 4]

Comparative Example 2 The procedure of Example 3 was repeated, except that no polycrystalline silicon was added to the nitric acid bath. In this etching, the etching amount is 3 as shown in Table 5.
It was about 1.1 g per hour.

[0033]

[Table 5]

[Brief description of the drawings]

FIG. 1 is a schematic view of a typical embodiment of the silicon etching method of the present invention.

FIG. 2 is a diagram illustrating a change with time of an etching amount measured in Example 1.

FIG. 3 is a diagram showing a change with time of an etching amount measured in Comparative Example 1.

[Explanation of symbols]

 1: container body 2: nitric acid bath 3: heating element 4: inner container 5: silicon

Claims (3)

[Claims] (1) producing nitric oxide or nitrogen dioxide in a nitric acid bath containing a reducing agent, and oxidizing the nitric oxide;
After producing nitrogen dioxide gas from the nitric acid bath, mixing the nitrogen dioxide gas and the hydrogen fluoride gas, etching silicon in the obtained nitrogen dioxide / hydrogen fluoride mixed gas or an aqueous solution thereof. Characteristic silicon etching method. 2. The method according to claim 1, wherein the hydrogen fluoride gas is vaporized from a hydrogen fluoride dissolving bath. 3. The method according to claim 2, wherein the nitric acid bath and the hydrogen fluoride dissolving bath are the same bath.